Oxidative stress is thought to play an important role in the initiation and progression of renal, cardiovascular, neoplastic, and neurodegenerative diseases. It is also widely believed that oxidative stress is a main cause of aging. Although considerable progress has been made in the understanding of the sources and actions of oxidative stress, the true role of oxygen-derived free radicals in the pathology of most human diseases largely remains to be determined. One major obstacle for radical research is the lack of specific and sensitive methods to quantify oxidative stress in vivo and in vitro. Although a multitude of different assays is available to assess free radical generation, each of these methods has substantial limitations. This article will provide a brief review on the most frequently used techniques to assess oxygen-derived free radical generation in isolated tissue preparations and cells. Emphasis will be put on most recent technical innovations and the shortcomings associated with current techniques.